This barrier is sometimes required to minimize the oxygen transfer into a closed loop heating system, protecting boilers and other accessories.Ī range of mechanical couplings, including compression, flare, and crimp ring, has been developed for PEX pipe joining. PEX pipe can be made with or without an EVOH (poly ) oxygen barrier as an intermediate/outer layer. Although this reaction is initiated during extrusion, the majority of cross-linking activity occurs at elevated temperatures when the tubing is placed in a sauna or immersed in water after extrusion is complete. During extrusion, the high temperature and moisture activate this agent, creating free radicals that link to other nearby polyethylene chains. Pre-extrusion treatment consists of grafting a cross-linking agent-such as silane and a catalyst-to the polyethylene chain. The die is heated to a high temperature where the cross-linking takes place because of a chemical reaction between the two. This process mixes peroxide with high-density polyethylene and feeds the mix into the extruder under high pressure. (Gamma radiation was the original method, but now electron-beam radiation predominates.) This tubing is then exposed to low-level radiation to energize the molecules and make them to link to their neighbors. Post-extrusion treatment starts by extruding polyethylene tubing with conventional equipment. Three Methods for Cross-Linking Polyethylene to 2-in.) diameters, SDR 9, and operating temperatures up to 82 C (180 F). All standards cover only outside, diameter controlled CTS-sized tubing of 3-mm to 51-mm (0.13-in. Most PEX tubing is made according to ASTM International F 876, Standard Specification for Crosslinked Polyethylene (PEX) Tubing, ASTM F 877, Standard Specification for Crosslinked Polyethylene (PEX) Plastic Hot-and Cold-Water Distribution Systems, and Canadian Standards Association (CSA) B137, Thermoplastic Pressure Piping Compendium (refer to B137.5). Some can be used continuously at operating temperatures as high as 93 C (200 F). Plastics Pipe Institute (PPI) TR-4, Hydrostatic Design Basis (HDB), Strength Design Basis (SDB), Pressure Design Basis (PDB) and Minimum Required Strength (MRS) Ratings for Thermoplastic Piping Materials or Pipe, lists PEX materials for various operating temperatures and maximum stresses. These traditional materials can also be difficult to cut and/or require specialized equipment, and may take a significant amount of time and expertise to assemble compared to PEX piping systems, which can be assembled in do-it-yourself homeowner kits.įigure 2 Closed and open loop heating system They often require open flames for effecting the joining systems for radiant heating applications. Traditional piping systems were rigid and difficult to work with because of their weight and the need for fittings at every turn. The heating profile is much more uniform, meaning fewer cold/hot spots (Figure 1, next page). This type of heating can provide superior comfort and efficiency compared to traditional forced-air convection heating. The heated surface then functions as a radiator, warming a room and all objects and people in it. Hydronic radiant floor heating employs heated water flowing through tubes or pipes under the floor. This temperature difference is the driving force behind radiant heat transfer. These other objects, in turn, radiate energy to other, cooler objects. Heated surfaces radiate energy, which is absorbed by other objects in a room. In North America, the pipe is rapidly gaining popularity for a variety of plumbing applications, including radiant heat. PEX pipe is already used extensively in Europe for underfloor heating. With the development of advanced materials in the 1960s-specifically PEX, or cross-linked polyethylene-this comfortable, efficient, and effective source of heat can be extremely reliable, durable, safe, and readily available. One of the more recent modern examples involves Frank Lloyd Wright, who incorporated hydronic radiant heat into his designs in the 1930s. Since the time of the Roman Empire, radiant systems have been used to deliver heat to buildings.
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